Process of preheating ores for reduction in smelting furnace



Dec. 21, 1965 F. c. COLLIN PROCESS OF PREHEATING ORES FOR REDUCTION INSMELTING FURNACE Filed Feb. 15, 1962 @Q QsQ M M 3: m i M ,m M mm w w W\I W 5? wt w W N m Na .n 3 W :M .wwwwfii H. m s Rd w United StatesPatent 6 Claims. to]. 7s 34 Treatment of ores in a rotary kiln attemperatures of about 900 to 1100 C. prior to reduction in an electricsmelting furnace is known and used in commerce. In the rotary kiln thecharge is heated by combustion of gas rich in CO which is formed byreaction between oxygen in the charge and a carbonaceous reducing agentand additional heat is ordinarily supplied at the discharge end of thekiln. The charge which is preheated and at times prereduced in therotary kiln is then fed into the electric furnace in which finalreduction and smelting take place.

While the rotary kiln has many outstanding advantages for pretreatingthe charge, heat transfer between the hot combustion gas and charge isnot particularly good and this is especially true at the relatively coolfeed end of the kiln. The layer of charge which moves alongthe bottom ofthe kiln has little if any contact with the hot combustion gas and onlythe surface layer is fully exposed to the heat of the gas. As a resultcommercial rotary kilns which may for example pretreat up to 10 to 50tons of charge per hour must be made very long and will average about 50to 150 meters in length. The temperature of the off gas leaving the feedend of the kiln will ordinarily be in the neighborhood of about 200 to500 C. and loss of heat in the off gas is a decided drawback toefficient operation. This heat loss is further increased when raw coalis fed directly through the wall of the rotary kiln into the hightemperature zone as this causes an increase in the temperature of thegas leaving the kiln. Feeding coal into the high temperature zone of therotary kiln is described in a copending application Serial No. 18,158,filed March 28,

1960 and now abandoned.

In accordance with the present invention, the heat content of the gasleaving the feed end of the rotary kiln is most effectively utilized bypassing the gas in countercurrent flow through a bed of charge movingdownwardly in a shaft. The hot gas in passing through the interstices inthe bed of charge contacts a large surface area of the charge to providea much greater transfer of heat than that obtained in the rotary kilnwithout sacrificing the advantages of pretreatment in the rotary kiln.

Best results are achieved by providing a plurality of relatively smallpreheating shafts which continuously feed parallel streams of chargeinto the rotary kiln. In general five to twenty shafts may be employeddepending upon the size of the equipment at hand and fans are preferablyused for sucking the hot gas through the bed of charge in the shaft. Insome applications it is of advantage to limit the height of the columnof charge in the shaft to about one meter.

As previously described the preheating shafts increase the heatefliciency of the rotary kiln operation and many other advantages may beachieved by the combined shaft and rotary kiln system of the presentinvention. For example when raw coal or other high volatile carbonaceousmaterial is fed into the high temperature zone of the rotary kiln theoff gases from the kiln may be at a temperature of about 500 to 900 C.or more. In such case the charge in the shafts may be heated to atemperature of about 500 to 800 C. before it is fed into the rotarykiln. Such a hot charge makes it possible to reduce the length of therotary kiln which is of advantage in reducing the cost of installation.The temperature of the off gas from the rotary kiln may be furtherincreased to about 1200 C. by feeding all of the carbon required forreduction of the charge as coal into the high temperature zone of therotary kiln. In such case the charge in the shafts may be heated toabout 1000 C. This is of particular advantage in connection with theproduction of calcium carbide as in such case the limestone will beheated and precalcined in the shafts while the final calcination andcoking of the raw coal will take place in the rotary kiln.

Many combinations are possible. A combination of preheating in theshafts and prereduction in the rotary kiln is especially advantageousfor treating pellets or briquettes. Raw pellets of finely ground metaloxide, bituminous coal and a binding agent such as Portland cement mustbe heat treated to provide the necessary mechanical strength forprereduction in a rotary kiln. In accordance with the present inventionthe pellets are heated treated in the shafts at a temperature of about600 to 900 C. At this temperature the coal is converted to coke andduring this process a coke lattice is formed which reinforces thepellets to provide the mechanical strength required. for treatment inthe rotary kiln. In the rotary kiln the pellets are heated to about 900to 1100 C. by combustion of gas in the space above the surface of thecharge to provide rapid and effective reduction of the oxides withoutany appreciable loss of carbon due to reaction between the oxygen in theair or combustion gas in the kiln. In the case of raw pellets orbriquettes it is especially advantageous to limit the height of the bedof charge in the shaft to about 1 meter because the raw pellets havesuch low mechanical strength that they tend to break under the pressureof a charge column of greater height.

In certain applications it is important to control the temperature andcomposition of the gas used in the shafts. In general this is done bycontrolling the amount of heat supplied at the discharge end of the kilnand by controlling the amount of air and coal fed into the hightemperature zone of the kiln. Final control of the temperature andcomposition of the gas in the'shaft is achieved by providing an air lockat the mouth of the shaft and by withdrawing the gas below the air lockby means of fans which may be employed for recycling the gas.

Further advantages and the details of the process of the presentinvention will be readily understood by reference to the accompanyingdrawings which illustrate one preferred form of apparatus for carryingout the present invention and in which FIG. 1 shows a side view of thecombined shaft and rotary kiln system of the present invention,

FIG. 2 is a section on line 22 of FIG. 1.

As illustrated in the drawings 10 is a rotary kiln with the usualrefractory lining 12 and roller supports 14 upon which the kiln rotates.Shaft 16 with refractory lining 18 is provided with a conventional typeair lock valve 20 for feeding charge into the shaft. Valve 22 controlsthe flow of charge from the discharge end of the shaft where the chargeis fed into the kiln by pipe 24. The off gas from the kiln is collectedin box 26 and fed into the bottom end portion of the shaft by the pipe28 and a conventional gas feeder 30 which distributes the gas to thecharge in shaft 16. The gas is collected at the top of the shaft by aconventional gas collector 32 and fan 34 overcomes the pressure drop inshaft 16 to suck the gas through the charge. Valves 36 and 38 controlthe flow of gas from the fan and the gas is either returned to theexhaust stack or recycled through pipe 39 to chamber 26 for adjustingthe temperature and composition of the gas fed to the shaft.

While only a single shaft 16 is illustrated in the drawings it will beunderstood that ordinarily a plurality of shafts will be employed forfeeding the charge in parallel flow into kiln 10. For best results eachshaft is equipped with its own individual fan and gas feeding andrecycle pipes connected to chamber 26 as described for shaft 16.

As shown in the drawing the rotary kiln may be equipped with one or moredevices for feeding coal or other high volatile carbonaceous materialinto the kiln. One such device is illustrated in the drawings and asthere shown the device comprises a hopper 40 with conveyor 41 whichfeeds coal into the open container 42. A plurality of pipes 44 whichextend into the interior of the furnace are bent at their outer end inthe direction of rotation of the kiln to provide scoops which pick upthe coal from container 42 and feed it onto the charge in the kiln whichis at a temperature of 800 C. or more. The heat of the charge willquickly drive off the volatiles and convert the coal to coke. Thevolatile material is burned above the charge with a luminous heatradiating flame.

Air for burning the volatiles may be introduced through the same pipesused for introduction of the coal or additional air pipes (not shown)may supply air to the kiln near the discharge end in conventional manneror excess air may be supplied by a burner if such is employed forintroducing heat into the discharge end of the kiln. If desired coke orother low volatile carbonaceous material may also be fed into the kilnby means of pipes 44.

The material treated in the kiln is discharged through a conventionalso-called front wagon 46 and it may be fed directly into an electricfurnace 48 by the inclined chute or pipe 50. In larger installation thekiln discharges into an intermediate hopper (not shown) and thedischarged material is then transferred from the hopper to the electricfurnace. In the structure shown in the drawings furnace gas rich in COis supplied to the discharge end of the kiln by pipe 52- and the gas isburned in the kiln to supply heat at this end of the kiln. If desired aconventional gas burned (not shown) may be used for supplying heat atthe discharge end of the kiln.

In a typical operation iron ore and the usual limestone flux are chargedto the preheating shafts and all of the carbon required for reduction ofthe charge is fed into the kiln as coal by pipes 44. Air fed into thekiln is so adjusted that the off gas of the kiln is at a temperature ofabout 1000 C. and contains about 3% oxygen. The iron ore and limestoneflux are heated to a temperature of about 900 C. in the shafts and as aresult FeS in the ore will be oxidized to Fe O and the limestone fluxcalcined to lime. Under these conditions about 30% of the oxygen isremoved from the ore in the rotary kiln. This illustrates the use of adivided charge in the system in which only the oxide ore and limestoneflux are fed into the shaft and in which all of the carbon required forreduction of the charge is fed into the kiln as raw coal. In such casean oxidizing atmosphere is provided in the shafts by supplying gascontaining from 1 to 5% oxygen and when the temperature of the charge inthe shafts reaches approximately 900 C. or greater sulphur is eliminatedfrom the ore and the limestone flux is calcined.

In another typical operation powdered iron ore 50 to 90% of which passedthrough a 200 mesh standard Taylor screen is pelletized with slightlymore than the stoichiometric proportion of bituminous coal necessary forreduction and about 5% Portland cement as a binder. The raw wet pelletswere hardened by storing for three days at atmospheric temperaturewhereby the point pressure strength of the pellets increased from 3 kgs.to 15 kgs. The mechanical strength of the hardened pellets of 15 kgs. isadequate for pretreatment in the shafts but insufficient for treatmentin the rotary kiln. The pellets are fed into the shaft.

In this case gas supplied by the furnace to the discharge end of therotary kiln contains 45% CO and 55% CO. The gas is burned in the rotarykiln and the supply of air is so controlled that the off gas from thekiln is at a temperature of about 900 C. The off gas contains about 13%CO 0.2% oxygen, 1.0% CO and the remainder N The pellets in the shaft areheated to about 800 C. by the hot off gas whereby the point pressurestrength of the pellets is increased from 15 kgs. to about kgs. toprovide the desired mechanical strength for treatment in the rotarykiln. The hardened pellets are transferred to the kiln where they areheated to a temperature of about 900 C. by combustion of the furnace gasand by combustion of CO generated by the prereduction reaction in thepellets. In the rotary kiln about 50% of the oxygen is removed from thepelletized ore. In the case of pelletized metal oxides the charge in theshaft is heated to a temperature of about 500 to 900 C. in order toprovide the pellets with the necessary mechanical strength for treatmentin the rotary kiln and for best results the oxygen content of the gasutilized in the shaft is below about 1.0% and the CO content is alsobelow about 1.0%. Care is taken to limit the depth of the charge columnin the shafts to about 1.0 meter because of the low mechanical strengthof the pellets charged to the shafts.

Excellent results are achieved using the system of the present inventionfor the production of carbide. In such case limestone is charged to theshafts while the raw coal is fed separately into the middle portion ofthe kiln. The combustion in the kiln is so adjusted that the off gas isat a temperature of about 1100 C. and contains about 5% oxygen. The gaswhen fed to the shafts will heat the limestone to about 1000 C. andcause it to be calcined to CaO. The temperature of the lime and cokeleaving the rotary kiln was about 1200 C. This example again illustratesthe use of a divided charge and in this case the temperature of thecharge in the shaft is raised to about 900 to 1200 C. to calcine thelimestone. Excess oxygen is supplied to the rotary kiln in order toachieve the heat required for calcining the limestone in the shafts inan oxidizing atmosphere.

It will now be understood that the system of the present invention maybe used for pretreatment of ores or other oxides or materials of thetype which are customarily treated in a submerged arc smelting furnace.These include iron oxides, various oxides for producing ferro alloyssuch as ferromanganese or ferro silicon and also can be used in theproduction of calcium carbide.

What I claim is:

1. The method of treating ores in a rotary kiln having at least oneshaft associated therewith for feeding charge into the kiln whichcomprises the steps of introducing charge into the shaft to establish acolumn of charge therein which is relatively short with respect to thelength of the kiln, moving the charge in the column downwardly in theshaft, collecting hot off gas from the rotary kiln, introducing at leasta portion of the hot off gas into the lower end portion of the shaft,passing the introduced off gas upwardly in the shaft in countercurrentflow through the interstices in the column of charge, withdrawing theintroduced off gas from the upper portion of the shaft and feeding thecharge heated by the introduced off gas into the rotary kiln.

2. The method specified in claim 1 which includes the step of recyclingat least a portion of the gas withdrawn from the upper portion of theshaft by feeding such portion back into the bottom of the shaft.

3. The method specified in claim 1 which includes the step of providinga flow of oxidizing gas countercurrent to the bed of charge containingfrom about 1 to 5% oxygen at a temperature of not more than 900 C.

4. The method specified in claim 1 which includes the step of providinga flow of gas which contains not more than about 1% oxygen, and not morethan about 1% CO, such gas being at a temperature of about 500 to 900 C.

5. The method specified in claim 1 which includes the step ofintroducing gas into the shaft at a temperature of about 900 C. to about1200 C.

6. The method of treating ores in a rotary kiln having a plurality ofstationary shafts associated therewith for feeding charge into the kiln,said method comprising the steps of introducing charge into the shaftsto establish vertical columns of charge therein which are relativelyshort in comparison with the length of the kiln, moving the charge ineach column downwardly, passing hot off gas from the rotary kilnupwardly through the interstices in each column countercurrent to themovement of the charge to heat the charge, withdrawing from each columnthe gas passed upwardly through the column, passing said withdrawn gasupwardly through the columns along with off gas from the rotary kiln andintroducing charge so heated into the rotary kiln.

References fitted by the Examiner UNITED STATES PATENTS Hornsey 75-36Gustafsson 7534 Halvorsen 75-34 Seil 26333 Kalling 7536 Wienert 7536Koniewiez 26333 Collin et al 75-11 DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, Examiner.

1. THE METHOD OF TREATING ORES IN A ROTARY KILN HAVING AT LEAST ONESHAFT ASSOCIATED THEREWITH FOR FEEDING CHARGE INTO THE KILN WHICHCOMPRISES THE STEPS OF INTRODUCING CHARGE INTO THE SHAFT TO ESTABLISH ACOLUMN OF CHARGE THEREIN WHICH IS RELATIVELY SHORT WITH RESPECT TO THELENGTH OF THE KILN, MOVING THE CHARGE IN THE COLUMN DOWNWARDLY IN THESHAFT, COLLECTING HOT OFF GAS FROM THE ROTARY KILN, INTRODUCING AT LEASTA PORTION OF THE HOT OFF GAS INTO THE LOWER END PORTION OF THE SHAFT,PASSING THE INTRODUCED OFF GAS UPWARDLY IN THE SHAFT IN COUNTERCURRENTFLOW THROUGH THE INTERSTICES IN THE COLUMN OF CHARGE, WITHDRAWING THEINTRODUCED OFF GAS FROM THE UPPER PORTION OF THE SHAFT AND FEEDING THECHARGE HEATED BY THE INTRODUCED OFF GAS INTO THE ROTARY KILN.